(1) Field of the Invention
This invention relates to a transmission unit and, more particularly, to a transmission unit for exercising transmission control over multiple bridges on a network.
(2) Description of the Related Art
In recent years transmission units having the bridge function of interconnecting multiple sites have been developed with the spread of the Internet. The scale and use of networks are extensively increased by the use of these transmission units.
Bridges operate at the layer 2 (data link layer) level and relay frames. (With Ethernet, these bridges can interpret Ethernet headers including a MAC address.) These bridges also function as basic interfaces with routers, switches, and the like.
Many of specifications for bridges are conformable to IEEE802.1D in which a spanning tree protocol is defined. This spanning tree protocol is used for avoiding frame congestion (circulation) caused by a loop by detecting a loop on a network, disconnecting the loop logically at a point (creating a blocked state), and forming a tree structure.
FIGS. 29 through 31 are views for giving an overview of a spanning tree protocol. Each of bridges a through c has bridge ID. It is assumed that the bridge IDs of the bridges a through c are 1, 2, and 3 respectively and that communication line speeds between the bridges a and b, between the bridges b and c, and between the bridges a and c are 100, 10, and 10 Mbps respectively.
First the bridges a through c exchange data called a bridge protocol data unit (BPDU) between them to recognize each other's bridge ID. Then a bridge with the smallest bridge ID will be selected as a root bridge. FIG. 30 indicates that the bridge a is selected as a root bridge.
After a root bridge is selected, designated ports (DP) and root ports (RP) are determined. A DP is the nearest port to the root bridge in each segment. The nearest port to the root bridge on each bridge is selected as an RP. These DPs and RPs are not blocked.
The segment between the bridges a and b will now be described with reference to FIG. 31. The nearest port to the root bridge (bridge a) is port #1 on the bridge a (because it connects directly with the root bridge). Similarly, between the bridges a and c, the nearest port to the root bridge is port #2 on the bridge a (because it also connects directly with the root bridge). Both ports #1 and #2 on the bridge a therefore are selected as DPs.
Of ports #1 and #2 on the bridge b, the port #1 is the nearer to the root bridge. Of ports #1 and #2 on the bridge c, the port #1 is the nearer to the root bridge. The port #1 on the bridge b and the port #1 on the bridge c therefore are selected as RPs.
The segment between the bridges b and c will now be described. The communication line speeds between the bridges a and b and between the bridges a and c are 100 and 10 Mbps respectively, so the segment between the bridges a and b is the shorter way to the root bridge. The port #2 on the bridge b therefore is a DP. The remaining port #2 on the bridge c which is not a DP or RP will be blocked. Blocking this port eliminates the loop structure of the entire network.
If there is a loop on a network, the loop structure will be eliminated in this way by the function of the spanning tree protocol.
The above conventional transmission units with a bridge function and IEEE802.1D, being a standard for them, are intended for the LAN. Therefore, even if a line is disconnected logically in compliance with a spanning tree protocol, the use of the disconnected line is not taken into consideration because the cost of that line is comparatively low.
However, lines forming a wide area network, such as a carrier network, are usually expensive and very valuable. If a line between transmission units with a bridge function used on such a carrier network is disconnected logically in compliance with a spanning tree protocol, it cannot be used for data transmission. This means a great waste and causes degradation in communication quality.